The flow conditions and the related stresses in glass and silastic model aneurysms located at bifurcations were quantitatively determined by means of laser-Doppler-anemometry. The flow velocities in straight terminal models with the aneurysm forming an extension of the afferent vessel were unstable if the outflow through the branches of the bifurcation was balanced. Average flow velocities in the fundus were small, but irregular flow fluctuations of high amplitudes were observed. Asymmetrical outflow through the branches of the bifurcation induced a rotatory intra-aneurysmal circulation from the dominant to the subordinate branch. The circulation in angled terminal aneurysms with the aneurysmal axis at a 45 degree angle to the plane of the bifurcation was a vortex caused by the eccentric inflow from the afferent vessel. Maximum flow velocities measured in the center plane of the angled terminal aneurysms were in the range 50%-80% of the axial velocity in the afferent vessel. The present results indicate that the geometrical relation between aneurysm and parent vessels is the primary factor governing the intra-aneurysmal flow pattern. The elasticity of the models did not affect the average flow velocities, but the intra-aneurysmal pulse wave was damped in elastic models. On the basis of the measured velocity gradients near the walls, maximum shear stresses on the wall of a typical human terminal aneurysm were estimated to be in the order of 50 dyne/cm2 (5 Pascal), a value that is similar to the shear stresses that occur at the flow divider of a cerebral artery bifurcation. This is based on absolute flow velocity measurements in patients [8, 13].